COGS 107B - Winter 2010 - Lecture 7 - Auditory System I
Cogs 107b – Systems Neuroscience www.dnitz.com lec7_01282010 – the auditory system I
basic characterization of sound waves frequency power (db)
outer ear (pinna) shape reflects sound from different sources in different ways providing a possible source for localization of the height of a sound source
middle ear bones amplify sound in a frequency-dependent fashion frequency-dependence of amplification is, in turn, modulated by middle ear muscle contractions frequency amplification (db)
basilar membrane vibration is transduced into neural signals by hair cells
the inner ear – transformation of pressure waves (sound) and their frequency characteristics (spectra) into neural signals
time amplitude low-frequency middle-frequency high-frequency time amplitude Fourier transform varying stiffness of the basilar membrane results in a Fourier transform of the vibrations of the endolymph
segregation of cochlear ganglion cell outputs to cochlear nucleus according to the position of their hair cell inputs – creation of a topographic representation of sound (tonotopy) that ultimately reaches primary auditory cortex
Hair cell deflections toward the kinocilium generate greater depolarizations than comparable deflections away from the kinocilium generate hyperpolarizations. Because of this, hair cells in regions of the basilar membrane oscillating at high frequencies exhibit non-oscillating depolarizations in response to high frequency sounds. Hair cell neurotransmitter release cannot increase and decrease at frequencies equal to those of sounds in the upper half of the audible range. Yet, the peaks and valleys of high frequency basilar membrane oscillations deflect hair cell cilia equal amounts and in opposite directions. How, then, does a hair cell signal the presence of a high frequency tone through neurotransmitter release?
cochlear nucleus neurons exhibit heterogeneous responses to inputs from ganglion cells. the response fields of each are described in frequency X amplitude response plots
ascending pathways of the mammalian auditory system
interaural time difference (ITD) interaural level difference (ILD) the ‘where’ of sound – sound source localization by comparison of inputs to the left and right ears not useful for persistent high frequency sounds (>2000 Hz) as hair cell responses do not oscillate in response to high frequency tones not useful for low frequency sounds as their amplitude is less impacted by the head what about sound source height?
brainstem processing of auditory information yields sound source localization the organization of cochlear nucleus outputs to the brainstem yields responses to interaural time differences in medial superior olive neurons and interaural level differences in lateral superior olive neurons.